This invention relates generally to gas turbine engines and more particularly to film cooled combustor liners used in such engines.
A gas turbine engine includes a compressor that provides pressurized air to a combustor wherein the air is mixed with fuel and ignited for generating hot combustion gases. These gases flow downstream to one or more turbines that extract energy therefrom to power the compressor and provide useful work such as powering an aircraft in flight. Combustors used in aircraft engines typically include inner and outer combustor liners that contain the combustion process and facilitate the distribution of air to the various combustor zones. The combustor liners are joined at their upstream ends to a dome assembly. The dome assembly includes an annular dome plate and a plurality of circumferentially spaced swirler assemblies mounted therein for introducing the fuel/air mixture to the combustion chamber. The liners facilitate air distribution by way of a number of dilution holes formed therein. The dilution holes introduce jets of air to primary and secondary zones of the combustion chamber. The dilution air quenches the flames so as to control the gas temperature to which the turbine hardware downstream of the combustor will be exposed. The quenching also reduces the level of NOx emissions in the engine exhaust.
Because they are exposed to intense heat generated by the combustion process, combustor liners are cooled to meet life expectancy requirements. Liner cooling is commonly provided by diverting a portion of the compressed air (which is relatively cool) and causing it to flow over the outer surfaces of the liners. In addition, a thin layer or film of cooling air is provided along the combustion side of the liners by directing cooling air flow through cooling holes formed in the liners. This technique, referred to as film cooling, reduces the overall thermal load on the liners because the mass flow through the cooling holes dilutes the hot combustion gas next to the liner surfaces, and the film of cooling air provides convective cooling of the liner walls. There are two basic types of liners that employ film cooling: multi-hole cooled liners and slot cooled liners.
Multi-hole cooled liners use a large number of very small cooling holes formed through the liners at a shallow angle (typically 20 degrees from the liner surface). Compressor air passes through the cooling holes to create closely packed, discrete jets of cooling air that coalesce and produce the film of cooling air on the combustion side of the liners. The cooling holes are generally distributed over the whole liner so as to provide a constant replenishing of the cooling film along the entire length of the liner. Slot cooled liners include a plurality of connected panel sections with a bump or nugget formed on the forward end of each panel section. An axially oriented slot is formed on the hot gas side surface of each panel section at the nugget, and a circumferentially disposed row of cooling holes is formed in the nugget. Compressor air passes through the cooling holes to produce the film of cooling air on the hot gas side surface of the panel section. Thus, the cooling film is replenished at each slot.
With either cooling approach, the difficulty in developing a successful liner design results from making appropriate thermal design trade offs between substrate temperature, surface temperature of and thermal barrier coating (TBC), bondcoat temperature, and thermal gradient through the TBC. Inadequate cooling can result in reduced low cycle fatigue life, increased oxidation rates of the TBC bondcoat and substrate, spallation of the TBC, and accelerated creep of the slot overhangs. Material selection and cross-sectional thickness (and hence weight) are also considered in designing liners. A multi-hole cooled liner typically requires a stronger substrate alloy or a thicker design, while a slot cooled liner benefits from the stiffening effect of the slot nuggets. However, the overall weight of a slot cooled liner is typically greater. It is also desirable to minimize the amount of cooling air needed for a liner design to increase engine efficiency and reduce emissions.
Both multi-hole cooled liners and slot cooled liners have proven to be effective for various applications. Multi-hole film cooling is particularly effective in continuous replenishment of an existing film and provides the added benefit of bore cooling of the liner substrate. However, cooling film volume is constrained by the spacing and size of the cooling holes. Slot film cooling is particularly effective in providing high volume cooling films in specific regions without being constrained by the hole size limitations of multi-hole film cooling. Historically, multi-hole film cooling uses less air to obtain acceptable substrate temperatures but is not as effective in cooling TBCs as slot film cooling. In addition, slot cooled liners tend to be more expensive and weigh more than comparable multi-hole cooled liners.
Regardless of the cooling approach, liners tend to develop hot spots or regions during operation. Different liner designs develop hot spots in different locations. Where hot spots occur can be a function of many factors including the configuration of the liners, dome assemblies and swirlers. For instance, the swirl of the combustion flow induced by the swirlers can cause hot gases to impinge against distinct regions of the liner surfaces. These regions tend to experience a loss of cooling film effectiveness and thus be more susceptible to thermal degradation. This effect, which is usually referred to as cooling film scrubbing, often occurs in the primary reaction zone of a combustor, although it can occur in other areas as well.
Hot spots are typically dealt with by providing sufficient total air flow to adequately cool the liner areas that would otherwise be susceptible to hot spots. However, this approach overcools non-problem areas, wasting cooling air and impairing engine efficiency. Accordingly, it would be desirable to have a combustor liner cooling scheme that adequately and efficiently cools all parts of the liner.
The above-mentioned need is met by the present invention, which provides a combustor liner having an annular shell which includes a first portion and a second portion. The first portion is provided with slot film cooling and the second portion is provided with multi-hole film cooling. The multi-hole cooling portion can be located either forward or aft of the slot film cooling portion, depending on the nature of the combustor that the liner is to be used in. In one possible embodiment, the liner includes a first annular panel, a second annular panel section joined at its forward end to the aft end of the first panel section, and a third annular panel section being joined at its forward end to the aft end of the second panel section. At least one of the panel sections has multi-hole film cooling and at least one other of the panel sections has slot film cooling.
The present invention and its advantages over the prior art will become apparent upon reading the following detailed description and the appended claims with reference to the accompanying drawings.